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Simulation of the Growth of Austenite from As-Quenched Martensite in Medium Mn Steels
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.ORCID iD: 0000-0002-0337-082X
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
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2018 (English)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 49A, no 4, p. 1053-1060Article in journal (Refereed) Published
Abstract [en]

As part of an ongoing development of third-generation advanced high-strength steels with acceptable cost, austenite reversion treatment of medium Mn steels becomes attractive because it can give rise to a microstructure of fine mixture of ferrite and austenite, leading to both high strength and large elongation. The growth of austenite during intercritical annealing is crucial for the final properties, primarily because it determines the fraction, composition, and phase stability of austenite. In the present work, the growth of austenite from as-quenched lath martensite in medium Mn steels has been simulated using the DICTRA software package. Cementite is added into the simulations based on experimental observations. Two types of systems (cells) are used, representing, respectively, (1) austenite and cementite forming apart from each other, and (2) austenite forming on the cementite/martensite interface. An interfacial dissipation energy has also been added to take into account a finite interface mobility. The simulations using the first type of setup with an addition of interfacial dissipation energy are able to reproduce the observed austenite growth in medium Mn steels reasonably well.

Place, publisher, year, edition, pages
Springer, 2018. Vol. 49A, no 4, p. 1053-1060
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-224673DOI: 10.1007/s11661-018-4497-3ISI: 000426686200006Scopus ID: 2-s2.0-85041509088OAI: oai:DiVA.org:kth-224673DiVA, id: diva2:1192791
Funder
VINNOVA
Note

QC 20180323

Available from: 2018-03-23 Created: 2018-03-23 Last updated: 2018-05-11Bibliographically approved
In thesis
1. Computational Materials Design of Medium Mn Steels
Open this publication in new window or tab >>Computational Materials Design of Medium Mn Steels
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Medium Mn steels (~ 3–10 mass % Mn), a new category of advanced high strength steels, attracted worldwide research interests recent years due to their excellent mechanical properties and low cost. These steels have fine microstructures and contain large fraction of metastable retained austenite (~ 30 volume %), therefore exhibit excellent strength and elongation. The fine microstructure is mainly introduced by an intercritical annealing process.

To accelerate the design of such steels, materials design is applied. The materials design concept is a systematic method. Contrary to conventional methods largely based on trial and error, it is based on the classical processing–structure–properties relationships and a quantitative knowledge of each relation represented by a mathematical model, so-called linkage model. Such models are thus an essential part in materials design.

The present thesis aims to develop a framework used for materials design of medium Mn steel. The development of models which serve as linkage tools is thus the focus. Tensile properties, i.e. strength and elongation, are set as the design objectives driven by the industrial application.

The major part is concentrated on the linkage tools of processing–structure, i.e. models and simulations to predict the microstructure evolution associated with processing. These linkage tools are based on thermodynamic calculations and kinetic simulations using the commercially available Thermo-Calc and DICTRA software. To be specific, the processing involves austenitization and quenching as well as intercritical annealing and quenching; while the associated structure involves transformation of austenite to martensite and reversion of martensite to austenite. Therefore the following aspects have been studied:

  1. martensite fraction with undercooling;
  2. austenite reversion during intercritical annealing;
  3. influence of austenite grain size on martensite start temperature;
  4. mechanical stability of retained austenite.

Besides these, prediction of tensile properties is studied in the last part, which serves as an example of a linkage tool of structure–properties.

Via integrating these models, to achieve certain tensile properties, the required microstructure and the associated processing can be traced back.

Abstract [sv]

Medium Mn stål (~ 3–10 mass% Mn), en ny typ av avancerade höghållfasta stål, har varit föremål för stort  globalt forskningsintresse de senaste åren på grund av stålens utmärkta mekaniska egenskaper och låga kostnader. Dessa stål har en fin mikrostrukturer och innehåller en stor fraktion av metastabil restaustenit (~ 30 volymprocent). Det uppvisar därför utmärkta värden på styrka och förlängning. Den fina mikrostrukturen åstadkommes huvudsakligen genom en sk interkritisk glödgningsprocess.

För att påskynda utvecklingen av sådana stål utnyttjas materialdesign. Materialdesignkonceptet är en systematisk metod. I motsats till konventionella metoder, som i stor utsträckning bygger på “trial and error”, baseras den på de klassiska relationerna process–struktur–egenskaper och en kvantitativ kunskap om varje relation representerad av en matematisk modell, en så kallad länkmodell. Sådana modeller är därför en väsentlig del i materialdesign.

Föreliggande avhandling syftar till att utveckla ett ramverk för materialdesign av medium Mn stål. Utvecklingen av modeller som fungerar som länkar är i fokus. Egenskaper vid enaxligt dragprov, dvs styrka och brottförlängning, formuleras som designkriterier i den givna industriapplikationen.

Avhandlinges huvuddel  koncentreras på länkmodeller mellan process och struktur, dvs modeller och simuleringsmetoder för att förutsäga mikrostrukturutvecklingen under värmebehandling. Dessa länkmodeller bygger på termodynamiska beräkningar och kinetisk simulering med hjälp av de kommersiellt tillgängliga Thermo-Calc- och DICTRA-koderna. Mer specifikt involverar värmebehandlingen austenitisering och släckning såväl som interkritisk glödgning och släckning; medan den associerade strukturen innebär omvandling av austenit till martensit och den omvända omvandlingen av martensit till austenit. Följaktligen har följande aspekter studerats:

  1. martensitfraktion som funktion av underkylning;
  2. austenit bildning under interkritisk glödgning;
  3. inverkan av austenitkornstorlek på martensitens starttemperatur;
  4. mekanisk stabilitet av restaustenit.

Förutom dessa aspekter analyseras förutsägelse av dragegenskaper i den sista delen, som exempel på länkmodell mellan struktur och egenskaper.

Genom att integrera dessa modeller, för att uppnå vissa dragegenskaper, kan den erforderliga mikrostrukturen och den därtill hörande behandlingen spåras tillbaka.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2018. p. 63
Series
TRITA-ITM-AVL ; 2018-27
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-227709 (URN)978-91-7729-801-4 (ISBN)
Public defence
2018-06-11, Sal B3, Brinellvägen 23, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
VINNOVA
Available from: 2018-05-15 Created: 2018-05-11 Last updated: 2018-05-15Bibliographically approved

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Huyan, FeiHöglund, Lars

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